Cold-rolling method for a wrought mg alloy with weak/non-basal texture and a cold rolled sheet

ABSTRACT

The present invention relates to a cold-rolling method for cold-rolling a wrought Mg alloy with a weak or non-basal texture as well as a cold-rolled sheet, the method comprising the steps of: pre-treating a billet of the wrought Mg alloy with a weak or non-basal texture, and then cold rolling it; wherein the weak or non-basal texture plane of said billet is selected as a rolling plane, and the rolling direction is parallel to the rolling plane; and said billet is cold rolled at room temperature to a sheet or foil with a thickness of 0.1 to 100 mm, wherein single-pass or multi-pass rolling is used, and the cold rolling is followed by an annealing at 200 to 400° C. for 10 min to 48 h.

TECHNICAL FIELD OF THE INVENTION

The present invention relates to a cold-rolling technique for manufacturing a cold rolled Mg sheet with excellent mechanical properties, which is featured to obviously enhance the strength of the wrought Mg alloy with weak/non-basal texture by the large stain cold-rolling.

BACKGROUND OF THE INVENTION

Compared with materials, such as plastics, woods, and other metals, magnesium and its alloys are more attractive candidates for a number of structural applications due to their high specific strength and stiffness, excellent damping capacity, good electromagnetic shielding, good machinability, and ease to be recycled. Therefore, they are progressively used in automotive, electronic and consumer electric appliance, leisure and gymnasium apparatus, bicycle, aviation, aerospace and defense industries. They are called “the Green Engineering Materials in the 21st Century”, and it can be expected that the consumption of Mg alloy as metallic structural materials will increase sharply in the near future, only inferior to the steel and Al alloy.

Due to the hexagonal close-packed (HCP) crystal structure of Mg and most of Mg alloys, the most common slip systems in Mg alloys are the basal slip system ({0001}<11-20>) with the Burger's vector of a/3<11-20> and the prismatic slip system ({1-100}<11-20>). The basal slip provides three geometrical slip systems, but only two of them are independent, while the prismatic slip system can hardly be activated at ambient temperature. Thus, Mg alloys has a poor ductility, workability and formability at middle to low temperature and room temperature, which limits their further press formability and industrial applications.

Thus, a large amount of work has been done to improve the room-temperature ductility of Mg alloys. Most of them focus on obtaining wrought Mg alloys with weak or non-basal textures by alloying elements, altering the processing method, or the like, so as to improve the room-temperature ductility thereof. CN 200810058278.9 provides a method of manufacturing Mg alloy sheet with high room-temperature ductility by using a hot-extruding process, wherein an extruding die with die holes having a specific shape is employed in the method. Owing to the asymmetric structure of the die hole, the shear stress is introduced during the extruding process, leading to the extruded sheet with a weak {0002} basal texture. Therefore, the Mg alloy sheet produced by this method exhibits excellent ductility at room temperature. For improving the low room-temperature ductility, distinct anisotropy and low strain hardening exponent of normal Mg alloy sheet, CN200910011111.1 provides a composition of a new Mg alloy sheet with weak texture and low anisotropy and with excellent room temperature formability, as well as the hot-rolling process for making a sheet therefrom. The hot-rolled Mg—Zn-RE sheet produced by the method shows a weak texture, high strain hardening exponent and low anisotropy, and the elongation-to-failure at room temperature is 30% to 45%, wherein the elongation-to-failure at room temperature along the rolling direction (RD) is 30 to 40%, and the elongation-to-failure at room temperature along the transverse direction (TD) is 35 to 46%, respectively. Although the wrought Mg alloys manufactured by those methods mentioned above with a weak or non-basal texture exhibit a high room-temperature ductility, their strength is very low; wherein the yield strength is lower than 160 MPa, and the ultimate tensile strength is lower than 230 MPa. It is not only far lower than that of Al alloy sheet for extensive use in the art, but also lower than that of the traditional rolled or extruded AZ31 Mg sheet (the yield strength is more than 180 MPa, and the ultimate tensile strength is more than 260 MPa). Thereby, the wrought Mg alloy with a weak or non-basal texture presents a low competitive power and its application is severely limited.

Cold-rolling process is mainly employed to produce thin sheets and belts, which can solve the problems that the size of the hot rolled sheet oversteps the tolerance and the mechanical properties in the same sheet are not uniform, due to the heat drop and non-uniform distribution of temperature during hot rolling. In addition, cold-rolled sheets usually have a high surface finish quality, excellent mechanical properties, and are rich in variety. The sheets with different surface roughness can be produced by cold rolling, and the products, produced by cold rolling with different reduction combined with the annealing treatment after cold rolling, may meet the requirement for further processing or application in a more comprehensive range. Nevertheless, the cold rolling response of the conventional wrought Mg alloy sheets, such as AZ31 rolled sheet, is generally poor due to its low ductility at room temperature, as shown in FIG. 1 (a), and it is easy to produce cracks in the surface or side margin when a reduction per pass is more than 10% at room temperature, as shown in FIG. 1 (b), thus it is improper for the conventional Mg alloy sheet to be cold rolled in commercial process, that is, a relatively large stain cold-rolling process.

SUMMARY OF THE INVENTION

To solve the problems found in the wrought Mg alloys with a weak or non-basal texture in the prior art, such as low strength, poor surface finish quality, the product size overstepping the tolerance, a thin sheet or foil having small thickness being not able to obtained therefrom, non-uniform microstructure and mechanical properties, the single use, and so on, the present invention provides a cold-rolling method for a wrought Mg alloy with a weak or non-basal texture. The strength of the wrought Mg alloy with a weak or non-basal texture can be enhanced by more than 15% by this cold-rolling method, as compared with that the billet of the wrought Mg alloy with a weak or non-basal texture for cold-rolling. Furthermore, the strength of the cold-rolled sheet, processed by proper annealing treatment, is increased by no less than 10%, and the elongation δ along the rolling direction is no less than 25%, compared with the billet of the wrought Mg alloy with a weak or non-basal texture for cold-rolling.

A technical solution of the invention is a cold rolling method for a wrought Mg alloy with a weak or non-basal texture having a weak or non-basal texture plane, comprising the steps of:

pre-treating a billet of the wrought Mg alloy with a weak or non-basal texture, and then cold rolling it in a rolling direction;

wherein the weak or non-basal texture plane of said billet is selected as a rolling plane, and the rolling direction is parallel to the rolling plane, and the thickness of said billet of the wrought Mg alloy with a weak or non-basal texture, perpendicular to the rolling plane, is in the range of 0.2 to 200 mm, and said billet is cold rolled at room temperature to the sheet or foil with the thickness of 0.1 to 100 mm.

Said wrought Mg alloys with a weak or non-basal texture is a Mg-RE-Zn system alloy, which contain (weight percentage) 0.1 to 10% RE, 0 to 5% Zn, ≦1% Zr and/or ≦2% Mn, with Mg being the balance.

RE is Gd, Y, or a mixture thereof.

When one of the planes of said billet is selected as a reference plane, wherein the maximum intensity of its (0002) pole figure is less than 6, or the position of the maximum intensity of its (0002) pole figure tilts no less than 25° from the normal direction of the reference plane, the selected plane is a weak or non-basal texture plane.

The grain size of said billet of the wrought Mg alloy with a weak or non-basal texture is no more than 100 μm.

The cold rolling is a single-pass cold-rolling process, having a reduction in the range of 20% to 50%, and a rolling speed in the range of 1 to 50 m/min.

The cold rolling is a multi-pass cold-rolling process, having rolling passes in the range of 10 to 50, a reduction per pass in the range of 1 to 5%, a total reduction in the range of 40% to 80%, and a rolling speed in the range of 1 to 50 m/min.

The cold-rolled sheets can be annealed at 200 to 400° C. for 10 min to 48 h.

Compared with the billet of the wrought Mg alloy with a weak or non-basal texture, the strength of the cold-rolled sheet is increased by no less than 15%.

Compared with the billet of the wrought Mg alloy with a weak or non-basal texture, the cold-rolled sheet after annealing treatment is increased by no less than 10% and the elongation δ along the rolling direction is no less than 25%.

The invention has the advantages and technical effects as follows:

1) In the invention, a large strain cold rolling is employed to process the wrought Mg alloy with a weak or non-basal texture at room temperature. High density of dislocations and a certain amount of twins can be introduced in the microstructure, and the intensity of basal texture component is increased during the cold-rolling process. Therefore, the yield strength and ultimate tensile strength of said billet are obviously enhanced. The cold rolled sheet has a relatively high strength at room temperature. The method is easy to manipulate and control.

2) By the proper annealing treatment after this cold-rolling process in the invention, the grain size of the cold-rolled sheet may turn to be finer, and its basal texture component may turn to be weak again, due to the partly or completely recrystallization during the annealing treatment. Compared with the original billet of the wrought Mg alloy with a weak or non-basal texture, the strength is greatly enhanced while ensuring a high ductility. Its mechanical properties may meet the requirement for further processing or application in a comprehensive range.

3) The original billet of the wrought Mg alloy with a weak or non-basal texture is eventually cold rolled to thin sheets or foils with the thickness of 0.1 to 100 mm in the invention. The strength of the cold-rolled sheet can be enhanced by no less than 15% along the rolling direction; then appropriate annealing treatment is adopted to the cold-rolled sheet, so the strength of the annealed cold-rolled sheet can be increased by no less than 10%, while ensuring a relatively high elongation (δ ≧25%) along the rolling direction, in comparison with the original billet of the wrought Mg alloy with a weak or non-basal texture. The strength of the cold rolled sheet is increased significantly, or the ultimate product of this invention exhibits high strength, accompanied with excellent ductility.

4) The cold-rolled sheets produced by this invention show high surface finish quality, and its final size, especially the thickness, easily meet the dimension tolerance.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1( a)-(b) show the (0002) pole figure of the AZ31 extruded sheet and its macrographs after cold rolling, wherein (a) shows the (0002) pole figure of the AZ31 extruded sheet, and (b) shows the macrographs of the AZ31 sheets after cold rolling.

FIG. 2( a)-(c) show the macrographs of GZ31 alloy specimens before and after cold rolling, wherein (a) shows the macrographs of GZ31 alloy specimens before cold rolling, where the billet for cold-rolling is a GZ31 hot-rolled sheets, and a plane formed by the rolling direction (RD) and the transverse direction (TD) of the sheet is selected as the reference plane, (b) shows the macrographs of the GZ31 alloy specimens after single-pass cold rolling to different thickness reduction, (c) shows the macrographs of the GZ31 alloy specimens after multi-pass cold rolling to the total thickness reduction of 45%, with reduction per pass of 1 to 5%.

FIG. 3( a)-(d) show the microstructure of the GZ31 alloy specimens before and after cold rolling, wherein (a) shows the microstructure of the GZ31 alloy specimen before cold rolling, (b) shows the microstructure of the GZ31 alloy specimen after single-pass cold rolling to the thickness reduction of 23%, (c) shows the microstructure of the GZ31 alloy specimen after multi-pass cold rolling, and (d) shows the microstructure of the GZ31 alloy specimen through the single-pass cold rolling followed by annealing treatment at 350° C.

FIG. 4( a)-(d) show the (0002) pole figure of the GZ31 alloy specimens before and after cold rolling, wherein (a) shows the (0002) pole figure of the GZ31 alloy specimen before cold rolling, (b) shows the (0002) pole figure of the GZ31 alloy specimen after single-pass cold rolling to the thickness reduction of 23%, (c) shows the (0002) pole figure of the GZ31 alloy specimen after multi-pass cold rolling, and (d) shows the (0002) pole figure of the GZ31 alloy specimen through the single-pass cold rolling followed by annealing treatment at 350° C.

FIG. 5( a)-(b) show the typical engineering stress-strain curves of the GZ31 alloy specimens before and after cold-rolling and of the cold rolled sheet after annealing along the rolling direction, wherein (a) shows the mechanical properties of the GZ31 alloy specimens before and after single-pass cold rolling to the thickness reduction of 23% and of the single-pass cold rolled sheet after annealing along the rolling direction, and (b) shows the mechanical properties of the GZ31 alloy specimens before and after multi-pass cold rolling and of the multi-pass cold rolled sheet after annealing along the rolling direction.

BEST MODES FOR CARRYING OUT THE INVENTION Example 1

1) A hot-rolled sheet of a Mg-3Gd-Zn (GZ31 Mg alloy), with a thickness of 3 mm, was selected as a billet. The chemical composition of the GZ31 Mg alloy was, by weight, 2.8% Gd, 1.1% Zn, and Mg being the balance, The elongation-to-failure δ of the hot-rolled sheet along RD was 35%, and the grain size was 20 μm, as shown in FIG. 3 (a). A plane formed by the rolling direction (RD) and the transverse direction (TD) of the hot-rolled sheet was selected as the reference plane, as shown in FIG. 2( a). The maximum intensity of its (0002) pole figure was 1.80, as shown in FIG. 4( a). Thus, the hot-rolled sheet was a wrought Mg alloy with a weak texture, and the selected reference plane of the billet was the weak texture plane.

2) The specimens with a dimension of 40 mm×25 mm×3 mm were cut out of the GZ31 hot-rolled sheets, and the surface thereof was polished with a waterproof abrasive paper to 800#, as shown in FIG. 2( a). Then they were rolled at room temperature (about 25° C.). The weak texture plane, i.e. the plane formed by the rolling direction (RD) and the transverse direction (TD) of the billet was selected as the rolling plane, and the rolling direction was parallel to the rolling direction of the original hot-rolling sheet. The rolling speed was 6 m/min, and single-pass rolling was adopted with the thickness reduction of 23%. The GZ31 cold-rolled sheet revealed macroscopically a good surface without cracks observed in the surface or side margin (except for the unstable rolling areas in the two ends), and a high surface finish quality, as shown in FIG. 2( b). The grains in the microstructure of the cold rolled sheet were elongated along the rolling direction, as shown in FIG. 3( b). The grain boundary was not so clear as that of the hot rolled sheet, as a result of the severe cold working, as shown in FIGS. 3( a) and (b). The basal poles of the majority of the grains in the GZ31 cold-rolled sheet switched back to ND, almost turning into a basal texture, and the maximum intensity increased to 2.88 (multiples of random distribution), as seen in the (0002) pole figure in FIG. 4( b), which meant the basal texture is strengthened.

3) Some cold-rolled sheets were performed an annealing treatment by maintaining them at 300° C. for 1 h or maintaining them at 350° C. for 30 min. No obvious recrystallization occurred when the GZ31 cold-rolled sheet was annealed at lower temperature (300° C.), and it should be the recovery taking place, in view of the metallographs of the microstructure of the cold-rolled sheets after maintaining them at 300° C. for 1 h. As the annealing temperature increased to 350° C., recrystallization took place substantially, a great deal of recrystallized grains could be detected clearly in FIG. 3 (d). However, the recrystallization occurred incompletely, thus most of the grain boundaries are not very clear. The size of the new grains was obviously smaller than that of the GZ31 hot-rolled sheet. The basal texture turned back into a weak texture with the texture intensity dropping to 1.9 m.r.d, as the cold-rolled specimen was annealed at 350° C. for 30 min (FIG. 4 (d)). Then, the cold-rolled and annealed product was trimmed, and covered with a protective film or coated with a protective oil, followed by packaging.

4) The engineering stress-strain curves of the GZ31 cold-rolled and annealed sheets as well as the hot-rolled sheet were presented in FIG. 5( a). It was evident from these curves that there was a significant increase in the strength of the cold-rolled sheet through single-pass rolling with the reduction of 23%, as compared with the GZ31 hot-rolled sheet. The 0.2% yield strength increased from 120 MPa to 300 MPa, which was far higher than that of the AZ31 sheet (the 0.2% yield strength, ultimate tensile strength and elongation-to-failure of commercial AZ31 hot-rolled sheet were usually ˜180 MPa, ˜230 MPa, and ˜12%, respectively), and the elongation-to-failure could reach to 16%. The tensile curve changed little, as the single-pass cold-rolled sheet was annealed at 300° C. for 1 h, and the 0.2% yield strength and ultimate tensile strength along RD are >270 MPa and >330 MPa, respectively. As it was annealed at 350° C. for 30 min, the elongation-to-failure δ along RD increased to ≧36%, meanwhile, the 0.2% yield strength and ultimate tensile strength were >200 MPa and >280 MPa, respectively. The strength of the cold-rolled sheet got an obvious enhancement without a great loss in ductility, in comparison with that of the original GZ31 Mg alloy hot-rolled sheet having excellent room temperature ductility.

Example 2

1) A hot-rolled sheet of a Mg-3Gd-Zn (GZ31 Mg alloy), with a thickness of 3 mm, was selected as a billet. The chemical composition of the GZ31 Mg alloy was, by weight, 2.8% Gd, 1.1% Zn, and Mg being the balance. The elongation-to-failure δ of the hot-rolled sheet along RD was 35%, and the grain size was 20 μm, as shown in FIG. 3 (a). A plane formed by the rolling direction (RD) and the transverse direction (TD) of the hot-rolled sheet was selected as the reference plane, as shown in FIG. 2( a). The maximum intensity of its (0002) pole figure was 1.80, as shown in FIG. 4( a). Thus, the hot-rolled sheet was a wrought Mg alloy with a weak texture, and the selected reference plane of the billet was the weak texture plane.

2) The specimens with a dimension of 40 mm×25 mm×3 mm were cut out of the GZ31 hot-rolled sheets, and the surface thereof was polished with a waterproof abrasive paper to 800#, as shown in FIG. 2( a). Then they were rolled at room temperature (about 25° C.). The weak texture plane, i.e. the plane formed by the rolling direction (RD) and the transverse direction (TD) of the billet was selected as the rolling plane, and the rolling direction was parallel to the rolling direction of the original hot-rolling sheet. The rolling speed was 6 m/min, and multi-pass rolling was adopted with the thickness reduction per pass of 1 to 5% and the total thickness reduction of 45%. The GZ31 cold-rolled sheet revealed macroscopically a good surface without cracks observed in the surface or side margin (except for the unstable rolling areas in the two ends), and a high surface finish quality, as shown in FIG. 2( c). As shown in FIG. 3( c), the microstructure of the cold-rolled GZ31 sheet through multi-pass roller was characterized by the presence of many bands diagonally inclined to the rolling plane, and a certain amount of twins were produced. These bands were formed in regions that have undergone {10-11} twinning followed by secondary {10-12} twinning. These structures were interpreted here as shear bands. The basal poles of the majority of the grains in the GZ31 cold-rolled sheet switched back to ND, almost turning into a basal texture, and the texture intensity increased to 3.38 (multiples of random distribution), as seen in the (0002) pole figure in FIG. 4( c), which meant the basal texture is strengthened.

3) Some cold-rolled sheets were performed an annealing treatment by maintaining them at 300° C. or 350° C. for 30 min. No obvious recrystallization occurred when the GZ31 cold-rolled sheet was annealed at lower temperature (300° C.), and it should be the recovery taking place, in view of the metallographs of the microstructure of the cold-rolled sheets after maintaining them at 300° C. for 30 min. As the annealing temperature increased to 350° C., recrystallization took place substantially, a great deal of recrystallized grains could be detected, and the size of the new grains was obviously smaller than that of the GZ31 hot-rolled sheet. The basal texture turned back into a weak texture with the texture intensity dropping to ˜2, as the cold-rolled specimen was annealed at 350° C. for 30 min. Then, the cold-rolled and annealed product was trimmed, and covered with a protective film or coated with a protective oil, followed by packaging.

4) The typical engineering stress-strain curves of the GZ31 cold-rolled and annealed sheets as well as the hot-rolled sheet were presented in FIG. 5( b). It was evident from these curves that there was a significant increase in the strength of the cold-rolled sheet through the multi-pass rolling with the total thickness reduction of 45%, as compared with the GZ31 hot-rolled sheet. As the multi-pass cold-rolled sheet was annealed at 300° C. for 30 min, the 0.2% yield strength and ultimate tensile strength along RD were >250 MPa and >310 MPa, respectively. As it was annealed at 350° C. for 30 min, the elongation-to-failure δ along RD increased to ≧38%, meanwhile, the 0.2% yield strength and ultimate tensile strength were >160 MPa and >230 MPa, respectively. The strength of the sheet got an obvious enhancement without a great loss in ductility, in comparison with that of the original GZ31 Mg alloy hot-rolled sheet having excellent room temperature ductility.

Example 3

1) A hot-rolled sheet of a Mg-2Zn-Y alloy, with a thickness of 3 mm, was selected as a billet. The chemical composition of the Mg-2Zn-Y alloy was, by weight, 1.9% Zn, 1.1% Y, and Mg being the balance. The 0.2% yield strength, ultimate tensile strength, and elongation-to failure δ of the hot-rolled sheet along TD were 122 MPa, 225 MPa, and about 26%, respectively. The grain size was ˜15 μm. A plane formed by the rolling direction (RD) and the transverse direction (TD) of the hot-rolled sheet was selected as the reference plane. The position of the maximum intensity of its (0002) pole figure tilted about 28° from ND. Thus, the hot-rolled sheet was a wrought Mg alloy with a non-basal texture, and the selected reference plane of the billet was the non-basal texture plane.

2) The specimens with a dimension of 40 mm×25 mm×3 mm were cut out of the hot-rolled sheets, and the surface thereof was polished with a waterproof abrasive paper to 800#. Then they were rolled at room temperature (about 25° C.). The non-basal texture plane, i.e. the plane formed by the rolling direction (RD) and the transverse direction (TD) of the billet was selected as the rolling plane, and the rolling direction was parallel to the TD of the original hot-rolling sheet. The rolling speed was 6 m/min, and single-pass rolling was adopted with the thickness reduction of 22%. The cold-rolled Mg-2Zn-Y sheet revealed macroscopically a good surface without cracks observed in the surface or side margin (except for the unstable rolling areas in the two ends), and a high surface finish quality. By the single-pass roller with the reduction of 22%, the grains in the microstructure of the cold rolled sheet were elongated along the rolling direction. The grain boundary was not so clear as that of the hot rolled sheet, as a result of the severe cold working. The basal poles of the majority of the grains in the cold-rolled Mg-2Zn-Y sheet switched back to ND, almost turning into a basal texture, and the texture intensity increased to 4.13, which meant the basal texture is strengthened.

3) Some cold-rolled sheets were performed an annealing treatment by maintaining them at 250° C. or 350° C. for 30 min. No obvious recrystallization occurred when the Mg-2Zn-Y cold-rolled sheet was annealed at lower temperature (300° C.), and it should be the recovery taking place. As the annealing temperature increased to 350° C., recrystallization took place substantially, a great deal of recrystallized grains could be detected, and the size of the new grains was obviously smaller than that of the Mg-2Zn-Y hot-rolled sheet. The basal texture turned back into a non-basal texture with the position of the maximum intensity of its (0002) pole figure tilting away from ND, as the cold-rolled specimen was annealed at 350° C. for 30 min. Then, the cold-rolled and annealed product was trimmed, and covered with a protective film or coated with a protective oil, followed by packaging.

4) The mechanical properties of the cold-rolled and annealed Mg-2Zn-Y sheets were summarized as follows. There was a significant increase in the strength of the cold-rolled sheet through single-pass rolling with the reduction of 22%, as compared with the hot-rolled Mg-2Zn-Y sheet. As the single-pass cold-rolled sheet was annealed at 250° C. for 30 min, the 0.2% yield strength and ultimate tensile strength along RD were >160 MPa and >270 MPa, respectively. As it was annealed at 350° C. for 30 min, the elongation-to-failure δ along RD increased to ≧25%, meanwhile, the 0.2% yield strength and ultimate tensile strength were >135 MPa and >250 MPa, respectively. The strength of the sheet got an obvious enhancement without a great loss in ductility, in comparison with that of the original Mg-2Zn-Y Mg alloy hot-rolled sheet.

Example 4

1) A hot-rolled sheet of a Mg—Y alloy, with a thickness of 3 mm, was selected as a billet. The chemical composition of the Mg—Y alloy was, by weight, 1.2% Y, and Mg being the balance. The 0.2% yield strength, ultimate tensile strength, and elongation-to failure δ of the hot-rolled sheet along RD were 134 MPa, 209 MPa, and about 39%, respectively. The grain size was ˜20 μm. A plane formed by the rolling direction (RD) and the transverse direction (TO) of the hot-rolled sheet was selected as the reference plane. The maximum intensity of its (0002) pole figure was 3.4. Thus, the hot-rolled sheet was a wrought Mg alloy with a weak texture, and the selected reference plane of the billet was the weak texture plane.

2) The specimens with a dimension of 40 mm×25 mm×3 mm were cut out of the hot-rolled Mg—Y sheets, and the surface thereof was polished with a waterproof abrasive paper. Then they were rolled at room temperature (about 25° C.). The weak texture plane, i.e. the plane formed by the rolling direction (RD) and the transverse direction (TD) of the billet was selected as the rolling plane, and the rolling direction was parallel to the RD of the original hot-rolling sheet. The rolling speed was 6 m/min, and single-pass rolling was adopted with the thickness reduction of 26%. The cold-rolled Mg—Y sheet revealed macroscopically a good surface without cracks observed in the surface or side margin (except for the unstable rolling areas in the two ends), and a high surface finish quality. By the single-pass roller with the reduction of 26%, the grains in the microstructure of the cold rolled sheet were elongated along the rolling direction. The grain boundary was not so clear as that of the hot rolled sheet, as a result of the severe cold working. The basal poles of the majority of the grains in the cold-rolled Mg—Y sheet switched back to ND, almost turning into a basal texture, and the texture intensity increased to 5.21, which meant the basal texture is strengthened.

3) Some cold-rolled sheets were performed an annealing treatment by maintaining them at 250° C. or 350° C. for 30 min. No obvious recrystallization occurred when the cold-rolled Mg—Y sheet was annealed at lower temperature (300° C.), and it should be the recovery taking place. As the annealing temperature increased to 350° C., recrystallization took place substantially, a great deal of recrystallized grains could be detected, and the size of the new grains was obviously smaller than that of the Mg—Y hot-rolled sheet. The basal texture turned back into a weak texture with the texture intensity dropping to ˜3, as the cold-rolled specimen was annealed at 350° C. for 30 min. Then, the cold-rolled and annealed product was trimmed, and covered with a protective film or coated with a protective oil, followed by packaging.

4) The mechanical properties of the Mg—Y cold-rolled and annealed sheets were summarized as follows. There was a significant increase in the strength of the cold-rolled sheet through single-pass rolling with the reduction of 26%, as compared with the Mg—Y hot-rolled sheet. As the single-pass cold-rolled sheet was annealed at 250° C. for 30 min, the 0.2% yield strength and ultimate tensile strength along RD were >180 MPa and >270 MPa, respectively. As it was annealed at 350° C. for 30 min, the elongation-to-failure δ along RD increased to ≧33%, meanwhile, the 0.2% yield strength and ultimate tensile strength were >150 MPa and >245 MPa, respectively. The strength of the sheet got an obvious enhancement without a great loss in ductility, in comparison with that of the original Mg—Y Mg alloy hot-rolled sheet. 

1. A cold-rolling method for cold-rolling a wrought Mg alloy with a weak/non-basal texture having a weak or non-basal texture plane, comprising the steps of: pre-treating a billet of the wrought Mg alloy with a weak or non-basal texture, and then cold rolling it in a rolling direction; wherein the weak or non-basal texture plane of said billet is selected as a rolling plane, and the rolling direction is parallel to the rolling plane; and the thickness of said billet of the wrought Mg alloy with a weak or non-basal texture, perpendicular to the rolling plane, is in the range of 0.2 to 200 mm; and when one of the planes of said billet is selected as a reference plane, wherein the maximum intensity of its (0002) pole figure is less than 6, or the position of the maximum intensity of its (0002) pole figure tilts no less than 25° from the normal direction of the reference plane, the selected plane is a weak or non-basal texture plane.
 2. The cold-rolling method of claim 1, wherein said wrought Mg alloys with a weak or non-basal texture is a Mg-RE or Mg-RE-Zn system alloy, which contains, by weight, 0.1 to 10% RE, 0 to 5% Zn, ≦1% Zr and/or ≦2% Mn, with Mg being the balance, wherein RE represents a rare earth metal.
 3. The cold-rolling method of claim 1, wherein RE is Gd, Y, or a mixture thereof.
 4. The cold-rolling method of claim 1, wherein the grain size of said billet of the wrought Mg alloy with a weak or non-basal texture is no more than 100 μm.
 5. The cold-rolling method of claim 1, wherein said billet is cold rolled to a sheet or foil with a thickness of 0.1 to 100 mm.
 6. The cold-rolling method of claim 1, wherein the cold rolling is a single-pass cold-rolling process, having a reduction in the range of 20% to 50%, and a rolling speed in the range of 1 to 50 m/min.
 7. The cold-rolling method of claim 1, wherein the cold rolling is a multi-pass cold-rolling process, having rolling passes in the range of 10 to 50, a reduction per pass in the range of 1 to 5%, a total reduction in the range of 40% to 80%, and a rolling speed in the range of 1 to 50 m/min.
 8. The cold-rolling method of claim 1, wherein the cold-rolled sheet or foil is optionally be annealed at 200 to 400° C. for 10 min to 48 h.
 9. The cold-rolling method of claim 5, wherein the cold-rolled sheet or foil is optionally be annealed at 200 to 400° C. for 10 min to 48 h.
 10. The cold-rolling method of claim 6, wherein the cold-rolled sheet or foil is optionally be annealed at 200 to 400° C. for 10 min to 48 h.
 11. A cold rolled sheet, of the wrought Mg alloy with a weak or non-basal texture, produced by the cold-rolling method of claim
 1. 12. The cold rolled sheet of claim 11, wherein compared with the billet of the wrought Mg alloy with a weak or non-basal texture for cold-rolling, the strength of the cold-rolled sheet is increased by no less than 15%, and compared with the billet of the wrought Mg alloy with a weak or non-basal texture for cold-rolling, the strength of the cold-rolled sheet after annealing treatment is increased by no less than 10%, and the elongation δ along the rolling direction is no less than 25%. 